1. Field of the Invention
The present invention relates to nonreciprocal circuit devices utilizing the Faraday effect.
2. Description of the Related Art
The main portion of a known nonreciprocal circuit device is shown in FIG. 8. A disc-like magnetic core 21 formed of ferrite, such as yttrium-iron-garnet (YIG), is disposed in a DC magnetic field generated by a permanent magnet (not shown), and the top surface of the magnetic core 21 is perpendicular to the direction of the DC magnetic field. Three central conductors 22, 23, and 24 are placed on the top surface of the magnetic core 21, and are held so that they overlap with each other at regular intervals (120xc2x0) substantially at the center of the magnetic core 21 while being insulated from each other. The lengths of the three central conductors 22, 23, and 24 are substantially the same, and thus, the inductances are also substantially equal.
The central conductors 22, 23, and 24 include two strip-like conductor portions 22a and 22b, 23a and 23b, and 24a and 24b, respectively, opposing each other. One end of the central conductor 22 serves as an input/output terminal 22c, and the other end thereof is used as a ground terminal 22c; one end of the central conductor 23 serves as an input/output terminal 23c, and the other end thereof is used as a ground terminal 23c; and one end of the central conductor 24 serves as an input/output terminal 24c, and the other end thereof is used as a ground terminal 24d. The input/output terminals 22c, 23c, and 24c are connected to corresponding circuits (not shown), and are grounded via matching termination capacitors 25, 26, and 27, respectively, which have equal capacitances. The ground terminals 22d, 23d, and 24d are connected to corresponding grounded casings (not shown).
The central conductor 22 and the termination capacitor 25 form a resonance circuit. Similarly, the central conductor 23 and the termination capacitor 26 form a resonance circuit, and the central conductor 24 and the termination capacitor 27 form a resonance circuit. The resonant frequencies of the resonance circuits are set by the corresponding termination capacitors 25, 26, and 27 so that they become equal to the frequency of an input signal. The central conductors 22, 23, and 24 are coupled to each other, and then, a double-tuned circuit is formed, for example, between the input/output terminals 22c and 23c. Similarly, double-tuned circuits are also formed between the input/output terminals 23c and 24c and between the input/output terminals 24c and 22c. 
In the above-described configuration, due to the Faraday effect, the following phenomenon occurs. A signal input into the input/output terminal 22c of the central conductor 22 is output to the input/output terminal 23c of the central conductor 23, which is displaced clockwise from the input/output terminal 22c by 120xc2x0. A signal input into the input/output terminal 23c of the central conductor 23 is output to the input/output terminal 24c of the central conductor 24, which is displaced clockwise from the input/output terminal 23c by 120xc2x0. A signal input into the input/output terminal 24c of the central conductor 24 is output to the input/output terminal 22c of the central conductor 22.
The central conductors 22, 23, and 24 overlap with each other on the magnetic core 21 such that they are extremely close to each other. Accordingly, the above-described double-tuned circuits are closely coupled to each other, and the transmission characteristic, for example, from the input/output terminal 22c to the input/output terminal 23c exhibits a double peak response, as shown in FIG. 9, and the insertion loss becomes large at frequency F0. The return loss indicating the input impedance or the output impedance at the input/output terminal also exhibits a double peak response, and becomes low (small) at frequency F0. For allowing the double-tuned circuits to critically coupled to each other, the central conductors can be separated by vertically displacing them from each other. It is difficult, however, to physically change the positional relationship among the central conductors in the vertical direction.
Accordingly, it is an object of the present invention to decrease loss at a signal frequency and to increase return loss at input/output terminals by ensuring a required transmission band for input/output signals.
In order to achieve the above object, the present invention provides a nonreciprocal circuit device including: a planar magnetic core disposed in a DC magnetic field, a top surface of the magnetic core being perpendicular to the direction of the DC magnetic field; and three central conductors disposed to overlap with each other substantially at a central portion of the top surface of the magnetic core, one end of each of the three central conductors being used as an input/output terminal, and the other end thereof being used as a ground terminal. The inductance per unit length from the central portion to the ground terminal of each of the three central conductors is set to be smaller than that from the central portion to the input/output terminal of each of the three central conductors.
With this configuration, the inductance from the central portion to the ground terminal becomes relatively smaller than that from the central portion to the input/output terminal. Accordingly, the transmission characteristic between the input/output terminals exhibits substantially a single peak response rather than a double peak response, thereby decreasing the transmission loss. The return loss also exhibits substantially a single peak response, and the impedance matching with another circuit connected to the nonreciprocal circuit device can be provided.
Each of the three central conductors may include two strip-like conductor portions opposing each other with an equal spacing therebetween, and a short-circuiting strip for connecting the two strip-like conductor portions may be provided between the central portion and the ground terminal. With this arrangement, the inductance from the central portion to the ground terminal becomes relatively smaller than that from the central portion to the input/output terminal.
Alternatively, each of the three central conductors may include two strip-like conductor portions opposing each other, and the width of each of the strip-like conductor portions from the central portion to the ground terminal may be set to be greater than that from the central portion to the input/output terminal. With this arrangement, the inductance from the central portion to the ground terminal becomes relatively smaller than that from the central portion to the input/output terminal. Additionally, a greater width of the strip-like conductor portions toward the ground terminal decreases the current loss.
Alternatively, each of the three central conductors may include two strip-like conductor portions opposing each other, and the spacing between the two strip-like conductor portions from the central portion to the ground terminal may be set to be greater than that from the central portion to the input/output terminal. With this arrangement, the inductance from the central portion to the ground terminal becomes relatively smaller than that from the central portion to the input/output terminal.
In the above-mentioned modification, the width of each of the strip-like conductor portions from the central portion to the ground terminal may be set to be greater than that from the central portion to the input/output terminal. With this arrangement, the inductance from the central portion to the ground terminal relatively becomes much smaller than that from the central portion to the input/output terminal. Additionally, a greater width of the strip-like conductor portions toward the ground terminal decreases the current loss.